I've never had to do it the way you mention it, nor would have had chosen to do it that way, but it's a question of culture and education, and we all receive different tips on how to behave with others, and feel differently when it comes to interact with others. This is just to say that I don't condemn what you say/think, I just say it's not what the etiquette or what many people recommend. And it's linked to the people's psychology and their way to deal with emotions.
We don't face truth, pain, sadness, joy, or reality, any kind of emotion, the same way. Most people, though, are hurt when a relationship ends. It seems important, then, to follow some "social" and "emotional" rules. Here are some useful links about that : How best to minimize the pain in a painful situation 1 and How to end a relationship the right way 2 - 3 - 4 - 5. It's not like you mentioned ("being a coward"), but rather about trying to minimize the pain you (may) inflict to the other person.
There are many reasons why you would chose not to do it over a message, but rather face to face. There are also a handful of ressources over the internet about that and a quick search will provide you with more links and data than what I gave you above. The important question is whether or not you want to soften the pain.
 In any breakup situation, the most important thing to remember is to be kind and compassionate.  Break up in person. It’s essential to be physically present to show that the relationship was important to you. Breakups by text may be common these days, but they hurt terribly and leave confusion in their wake.
You can't easily do that over text or email, as emotions and feelings aren't clearly identified or can be missed or misinterpreted, as explained here by Berkeley graduate Tchiki Davis : Reading Emotions in Text Messages.
Text messaging can breed disastrous misunderstandings between people.
When you have read about the topic, you can make a more informed choice I believe.
FWIW : I've done many mistakes in the past (I was then much younger) when it came to deal with such situations (at work or with acquaintances, but never with a GF/friend), and I've been the "coward". I now feel much better when I face people and tell them what I have to. In any situation. My advice would then be to do it in person.
#3 - relationships end for a wide variety of reasons. Conflict is one common reason, but sometimes it involves other reasons that mean ending a relationship with someone you still care about. When this happens, you have to learn how to break up with someone you love
#4 - ending a relationship is one of the most difficult things we have to do. No matter where you are in the breakup process, knowing how to break up well (including how to break up with someone you love) can help make this transition smoother and less harmful for both partners.
#5 - if you've ever been dumped by text or email (or if you've been ghosted altogether), you know how it feels to be given so little consideration that the other person didn't even bother to tell you in person. Why do the same to another person?